1. Field of the Invention
This invention relates to a method of measuring heat capacity of a sample, and more particularly to a method of measuring an absolute value of heat capacity of a sample by AC calorimetry with high accuracy.
2. Related Art Statement
AC calorimetry has been well known as a method of measuring heat capacity of a liquid sample, in which the liquid sample is periodically heated in an alternate manner and the heat capacity is measured by detecting a temperature modulation of the liquid sample. In the past, heat capacity of a liquid sample was measured by filling a sample cell formed by two opposing plates with the liquid sample. However, in this method, it is very difficult to keep a distance between the two plates constant, and thus measuring accuracy is limited. Moreover, alternate temperature of the vacant cell cannot be precisely measured because two plates are not thermally contacted with each other when the sample cell is not filled with the liquid sample. In this manner, heat capacity of the sample cell itself cannot be measured accurately, and this results in that an absolute value of heat capacity of the liquid sample cannot be measured.
To remove these defects, the inventors of the present application have developed an improved light irradiation type AC calorimetry, which has been described in "Heat and Temperature Measurement and Thermal Analysis", Heat Measurement Research, pp. 74-81, 1973, "Japanese Journal of Applied Physics", Vol. 20, No. 11, pp. 1995-2011, November 1981, and Utility Model Publication of Application 5-14200.
In the heat capacity measurement by the known light irradiation type AC calorimetry of the above mentioned Utility Model Application Publication 5-14200, a sample cell is formed by arranging a very small tube having a very thin wall and a very small diameter in a thermal bath such that its both ends are supported by the thermal bath. A liquid sample contained in the tube is heated in an alternating manner, for example, at 0.2-10 Hz repetition frequency by irradiating light intermittently to this tube by means of a chopper from outside of the thermal bath, and a temperature modulation (called AC temperature) of the liquid sample is then detected by the thermo-sensor provided on surface of the tube. Then, an amplitude of the thus detected AC temperature is measured. Prior to or after the above measurement, amplitudes of AC temperatures are detected for the vacant tube in the sample cell as well as for the tube filled with a standard liquid having known heat capacity and density. Thus, the heat capacity of the liquid sample is measured or derived from the amplitudes of these three AC temperatures. This light irradiation type AC calorimetry is based on a fact that an AC component of the temperature change in the liquid sample, i.e. the amplitude of AC temperature, is in inverse proportion to the heat capacity of the liquid sample when the liquid sample is heated periodically.
In the above mentioned light irradiation type AC calorimetry, it is advantageous that the absolute value of the heat capacity of a liquid sample can be measured much more precisely. But recently, it has been required to develop a method which can measure a slight change in heat capacity of a solution having a very small amount of substance dissolved therein. However it has been confirmed that such a requirement could not be satisfied with the light irradiation AC calorimetry. The reason is the following.
It is confirmed that a measurement error caused by heat leakage is substantially large in the known light irradiation type AC calorimetry. In the AC calorimetry, it is necessary to consider the heat leakage. However, in practice, it is very difficult to perform the AC calorimetry while the heat leakage is corrected. Therefore, in the known AC calorimetry, a condition of 1/.tau..sub.e &lt;.omega. is satisfied so that a correction term of 1/.omega..sup.2 .tau..sub.e .sup.2 can be neglected. Herein .omega. is an angular frequency of AC heat flow, and .tau..sub.e is an external relaxation time, which is a physical amount defined by a product between heat capacity of a sample and thermal resistance between the sample and thermal bath. Accordingly, the need for compensating the heat leakage is avoided by making a speed of the heat leakage from the tube to the thermal bath less than the AC heat period. Thus, the heat capacity has to be measured by setting a lower limit for the repetition frequency of light irradiation. However, it is required to make the sample cell tube as small as possible, because the liquid heat diffusion rate is not large. Then, a ratio of a volume to a surface area of the sample cell tube becomes small and heat is liable to leak, and therefore measurement error is involved in a measured heat capacity. Moreover, an amount of heat leak differ slightly depending upon kinds of liquids and filling conditions of the sample cell tube, i.e. whether the tube is filled with a sample or not.
Due to the above explained problem, the measurement accuracy of the heat capacity of the sample is at most .+-.0.2% in the known light irradiation type AC calorimetry. However in recent technological trend, it has been required keep the measurement accuracy of the heat capacity of the liquid sample less than .+-.0.2%. It is apparent that such a requirement could not be satisfied.
Furthermore, in the known AC calorimetry, a sample is heated in a periodic manner by periodically cutting off a the light beam emitted from a halogen lamp with the aid of a chopper. However, light intensity of the light source is not strictly constant, and generally decreases with time, and thus the light irradiation heating is not suitable for the measurement in which irradiation time has to be prolonged in order to raise the measurement accuracy. In this manner, the measurement error is increased due to light intensity. Moreover, the known light irradiation type AC calorimetry has a defect in that a large size light source and a large size light chopper are needed, so that the light source equipment is liable to be complicated and large. Furthermore, it is necessary to provide an opening in the thermal bath in order to pass the irradiation light onto the sample cell tube. It is apparent that such an opening decreases the faculty of the thermal bath and gives a possibility of introducing another measurement error.